Interior door liners and other components for automobiles are manufactured using an injection molding process. Thermoset material can be injection molded to a rigid part and then subsequently a textured interior surfacing material is attached to give the liner a textured surface. However, such process is time consuming and difficult to maintain adequate quality controls to be economical.
It is possible to mold the thermoset material directly to the textured surface. U.S. Pat. Nos. 5,110,532 and 5,151,237 disclose methods of injection molding a flat deformable laminate material. Thermoset resin is injected into a chamber through a single gate and then about a deformable material onto a soft porous material to form a unitary laminate bonded together by the thermoset material. The laminate can then be heated deforming the thermoset material to a desired shape.
In U.S. Pat. No. 4,743,323, another method of injection molding a composite article is disclosed. Fabric material is placed within the molding cavity and maintained under tension during the molding operation. Thermoset material is injected through a single gate, through a multiple port header into the molding cavity from opposite ends thereof.
The prior art processes generally utilize a relatively high pressure machine to undertake the injection process. The full tonnage of the machine is used for each injection cycle to maintain the mold sections together. As well, maximum pressure is used for injecting the thermoset material into the mold. Maximum pressures are required to ensure that the cavity becomes fully filled with the thermoset material. However, the use of maximum pressures is inefficient and tends to reduce the operational lifespan of the machine.
Apparatus of the prior art generally have a stationary mold section and a moveable mold section secured to a base plate that is movably supported on slide or guide rods in response to the operation of a hydraulic cylinder mounted on a ram. The ram passes through the base whereupon a disc swings to cover the opening of the base plate providing a foundation for the ram. The hydraulic cylinder extends urging the mold sections together. The useful travel of the hydraulic cylinder is very limited thus limiting the depth of a mold which can be used and ultimately the size of the part which can be molded. Such apparatus are commercially available under the trademark BATTENFELD.
Apparatus of the prior art generally undertake a single injection of melt into a mold cavity. For simple molded shapes, a single gate produces adequate results. However, for more complicated shapes, multiple controlled gates must be used in order to ensure that the melt fully occupies the mold cavity. Higher injection pressures are used to ensure complete injection. Higher injection pressure necessitates higher press pressure to maintain the mold sections together containing the injected material.
Recently, controlled low-pressure molding has been recognized as a viable technology. In controlled low-pressure molding, a fully plasticized and uniform melt is injected into a mold cavity while speed and injection pressure are controlled to assure that the melt front or leading edge remains intact throughout the filling process. This process eliminates the need for packing out the product reducing the material used and molded-in stresses. Controlled low-pressure molding permits a melt temperature which is lower than conventional high speed and high pressure molding. Further, larger gates with positive displacement shut off valves are used to minimize frictional heat and shear in the melt.
The reduced pressures on injection reduces the pressure differentials within the molded part. Pressure differential affects the shrinkage. In a typical high pressure injection, the gate pressure can be about 20,000 psi and a cavity pressure at end-of-fill can be about 18,000 psi. In contrast, low-pressure molding experiences an injection pressure of 6,000 psi and an end-of-fill pressure of 5,000 psi.
Low-pressure molding provides a method for producing more complicated parts using a single injection process. However, new machinery to replace existing high pressure apparatus is expensive. Although high pressure apparatus can be retrofitted to operate as a low-pressure molding apparatus, without extensive rebuilding, the high pressure machines cannot accommodate larger mold sections used for molding complicated parts.